The commercially available rechargeable electrochemical battery system with the greatest stored energy per unit weight and volume is based on the well-known silver-zinc couple. Since the silver plate is quite expensive, use of this battery system is restricted to applications where the energy density of the battery is critical to the total system mission. An example of such an application is the propulsion system power source for underseas vehicles.
Since the commercial introduction of this battery, about twenty years ago, several hundred million dollars has been spent on research and development of the engineered product, which still has great reliability problems. Silver-zinc cells often spontaneously exhibit internal electrical shorts due to breakdown of the separator material caused by silver oxidation and zinc dendritic growth. Both plates are somewhat soluble in the electrolyte which indicates an undesirable thermodynamic instability.
The zinc plate on recharge has a tendency to grow sharp dendritic, needlelike, crystals which penetrate the separators. The smallest deformation or fracture in the separator is a path through which a zinc dendrite can form. A path also exists if the electrolyte level is above the top of the separators. This means that a silver-zinc battery must remain in essentially a vertical position. Even in the best case, the zinc electrode changes shape by slumping on cycling, causing drastic loss of cell capacity in a few tens of cycles.
In another very serious associated problem, a conducting silver film tends to deposit on the separators. On charge, silver peroxide is formed on the silver plate. This is slightly soluble in the electrolyte and will form perhydroxyl ions in solution which chemically attacks the cellulose of the standard cellophane separator systems. Silver can then deposit on the zinc plate causing deleterious effects on the battery system.
This poor life and reliability of the silver-zinc system has impeded its use in many applications and restricted the type of missions the battery system could engage in. Fletcher, in U.S. Pat. No. 3,790,409, tried to solve these problems by using a separator system consisting of several polyethylene-methacrylic acid semipermeable membranes sandwiched between a single polypropylene electrolyte absorber sheet and a multiple layer of regenerated cellulose as the prime electrolyte absorber material. Arrange, in U.S. Pat. No. 3,671,319, preferred inorganic separators, such as zirconia or magnesium-iron silicates.
Ameln, in U.S. Pat. No. 2,677,006, taught a battery cell comprising porous anodes made of sintered nickel powder particles supported by a nickel or nickel plated iron net. The positive plate was provided with a silver oxide active battery material. The negatives plates were made of pockets of perforated iron sheet plated with nickel, or pockets of perforated iron or nickel sheet. The active battery material for the negative plate consisted of iron, iron-cadmium, or iron-mercury oxide, which was contained within the pockets of the rather bulky, heavy structure.
Brown et al, in U.S. Pat. No. 3,853,624, taught a high energy density battery based on an iron-nickel couple, utilizing a porous, metal fiber plate construction. This type of battery provides excellent energy density values of about 24 watt-hours/pound and 1.8 watt-hours/cubic inch of cell.
None of these battery systems, however, provides the necessary long life, reliability, light weight, compactness, and energy density values, of about 35 watt-hours/pounds and 2.5 watt-hours/cubic inch of cell, up to about 200 deep discharge-charge cycles, required for use, such as an alternate power source in submarines, where cost is of secondary importance.